This text book chapter will focus on discovering where syntax is localized within the human brain. When taking a first year Psychology course, most people learn about Broca's area and the type of aphasia associated. However, syntax is a complicated subject and recent studies have shown that syntax is not solely situated within Broca's area. Instead it appears that syntax has many various properties and uses different areas throughout the brain to process different aspects of syntax, and by harnessing the different regions in your brain it is able to conduct the exhaustive task of decoding syntax.[1]

This chapter will lead you through the history of how syntax was localized within the brain, as well as provide information about how these older views have been replaced through new neuroimaging technology that is now available. Models of syntactic processing will also be discussed.

In the 19th century, Franz Joseph Gall proposed the "only true science of mind": phrenology.[2] Phrenology was composed of basic tenents, such as "the brain is the organ of the mind", etc. Gall believed that the mind was separated into individual faculties, and that these faculties had to be separated from one another within the brain. Interestingly Gall believed that the size of the brain was equivalent to power. Gall also believed that the shape of the brain was determined by the development of the separate mental faculties. Since the skull formed its shape based on the shape of the brain, Gall theorized that the surface of the skull could be read as a way to measure psychological tendencies and aptitudes.

Thus a tradition was born within the 19th century that by examining the skull of an individual, you could discover their particular intellectual aptitudes and traits. If an individual had a prominent ridge in the forehead this indicated that they would exhibit benevolent behaviour. Many phrenologists however only looked for subjects that confirmed their hypotheses, and ignored those who contradicted them. Sometimes counter-evidence was explained away so that perhaps someone who did have a large benevolent organ may have other organs that counteracted the organ.

It wasn't until the 1860's when another scientist demonstrated evidence that the popular science of phrenology may be wrong.

In the 1860’s a French surgeon laid the foundation of cognitive neuroscience and modern neuropsychology with the discovery of an individual (Leborgne) suffering from neurological problems who could no longer respond to questions or produce any type of speech.[3] After the death of Leborgne, Broca performed an autopsy and discovered a lesion on the left frontal lobe. A few months after the discovery of Leborgne, a new patient, Lelong, came to Broca and he had a stroke earlier in the year and also suffered a neurological ailment but unlike Leborgne he was able to produce speech. After an autopsy, Broca discovered that Lelong also had a lesion on the left frontal lobe.The region that Broca identified is now known as Broca’s area, and the neurological problems that come from a disturbance within Broca’s area are typically known as Broca’s aphasia.

Paul Broca was one of the first scientists to conclusively demonstrate that different brain areas can specify a function. This was in contrast to phrenologists who claimed that brain areas were specific to faculties, but never showed any real data that conclusively proved their claims. Lelong and Leborgne both had lesions within the left frontal lobe but Lelong was able to still communicate unlike Leborgne. This difference helped Broca demonstrate that lesions needed to be very specific in location in order to affect speech production.

A few years after Broca’s discovery, Carl Wernicke had a patient who suffered from a stroke in 1873.[4] Wernicke’s patient was able to speak, and while his hearing was not impaired he was unable to understand other people who spoke to him. After the patient passed, Wernicke performed an autopsy and examined the brain. Wernicke discovered a lesion in the left hemisphere of the brain, on the rear temporal/parietal region and concluded that this region was involved in speech comprehension. In Wernicke’s aphasics, it has been observed that they are capable of producing sentences with correct syntax, which suggests that certain brain regions are responsible for syntax processing.[1] Wernicke's discovery contributed to the idea that areas within the brain perform specific functions since he discovered an aphasiac separate from Broca's aphasiacs that had a lesion within the left temporal/parietal lobe.

People who suffer from aphasia typically have trouble with either forming speech or understanding speech.[5] There are two different categories of aphasia: non-fluent and fluent aphasia. People who suffer damage to the left frontal lobe typically suffer from non-fluent aphasia (Broca’s aphasia). Symptoms include problems with articulation, repetition of words, speech production, speech comprehension, fluency, and problems with finding appropriate words.[3] People who suffer Broca's aphasia typically speak in short phrases that make sense but take great difficulty to product.[5]Some patients suffer from more severe forms of the disorder in that they are only able to articulate words or phrases that are always repeated whenever they try to speak, as in the case of Leborgne who could only ever say “tan”. [3] However, people with Broca's aphasia are able to understand others and thus are aware of their own mistakes.[5] People with Broca's aphasia typically also suffer from weakness on their right side of their body. Sometimes they may be paralyzed and this is because the frontal lobe also controls motor movements.

Another type of non-fluent aphasia is global aphasia. Global aphasia is a result of extensive damage in the language centres of the brain. People who suffer from global aphasia have severe difficulties with speech and comprehension.

Typically damage to the left temporal lobe causes Wernicke's aphasia which is a fluent aphasia. However damage to the right temporal lobe can also sometimes result in Wernicke's aphasia. People who suffer from Wernicke's aphasia tend to speak in dragged out sentences that have no meaning, sometimes made up words, and sometimes unnecessary words are used as well. It's often difficult to understand what someone with Wernicke's aphasia is trying to say, and people with Wernicke's aphasia have difficultly understanding other people as they are unaware that make mistakes.

After Broca and Wernicke's discoveries and as technology advanced, new techniques were used to try and localize the exact regions that were critical for syntax processing. Through Event-Related Brain Potentials (ERPs), MRI’s, fMRI’s, and MEG scans researchers have been working diligently to localize these regions.

Researchers began to examine syntax and semantics into further detail.[6] Evidence suggested that there was a distinct difference between semantic and syntactic processing. A study by Freedman and Forster (1985) who set out to study the processing effects on ungrammatical violations.[7] They showed that there were different processing systems involved in syntax processing (i.e. gender information, word category information, etc). After this study came out Neville et al (1991) set out to discover whether syntactic processing was separate from other aspects of language processing through the use of ERP measurements.[6] At this point in time, the only ERP measurement found from language studies was an N400 (a centro-parietal negativity within the brain that occurred around 400 msec) that was reported to be involved in semantic processing. While some studies had shown that a later P600 (a late centro-parietal positivity that occurred around 600 msec) was observed during syntactic measurements, it had not yet been conclusively shown to be related to syntax. Neville et al (1991) formed their study to include sentences that were semantically violated but with proper syntax, or with violated syntax and proper semantics. They set up their experiment in such a way so that the N400 associated with semantics could be seen as well as any other distinct ERP patterns that would perhaps be involved in syntax. They discovered that when a sentence had deviant syntax ERP measurements were recorded that were separate from the N400. These ERP measurements were an N125 (an anterior left hemisphere negativity at 125 msec), which seemed to be involved in detecting syntactic violations, and a P250 (left hemisphere positivity around 250 msec). None of the measurements associated with syntax were observed with a N400. Neville et al (1991) showed through ERP that syntax processing was separate from semantic processing.

Neville et al (1991) had shown through ERP measurements that there were ERP measurements associated with syntax, and that these measurements were separate from semantic processing.[6] Friederici et al (2003) set out to demonstrate that semantic processing at the sentence level, and syntactic information processing involved different systems through ERP measurements.[8] They also found that semantic processing activates a centro-parietal negativity that occurs approximately around 400 ms (N400) after being shown a sentence.[8] This potential is capable of varying due to the different variations in semantic processing, such as lexical-semantic information, lexical status, pragmatic information, and thematic information. However when Friederici et al (2003) compared the N400 with syntactic processing ERP measurements, syntactic processing has an early and late ERP component (left anterior negativity; ELAN) that is between 140-400 ms. A late centro-parietal positivity is also associated with syntactic processing after 600 ms (P600). Friederici et al (2003) proposed that these two ERP components associated with syntactic processing are related to two different syntactic processes: an initial process that is involved in automatically structure-building sentences, and a later process which is involved in controlling the reanalysis and repair of syntax.

After concluding that syntax and semantics were separate from one another, Friederici et al (2003) used functional magnetic resonance imaging (fMRI) to try and localize the areas involved. fMRI analysis localized the N400 ERP from semantic processing within the brain: the hippocampus may be involved, cortical areas within the superior temporal sulcus, and the left auditory cortex. When using fMRI to analyze the ELAN involved in syntactic processing it was found that the ELAN was localized within the anterior temporal and inferior frontal cortices in both hemispheres, but there was left hemisphere dominance. Friederici et al (2003) designed an experiment using anomalous sentences (abnormal sentences) in order to localize the regions involved in the P600. Friederici et al's (2003) study did identify that the left basal ganglia of the putamen was associated with the P600 potential. It was found to be involved in the later controlled syntactic processing rather than with the early structure building processes. This study did show that there was a difference between semantic and syntactic processing, as well showed the different regions within the brain associated with the two ERPs involved in syntactic processing. Latter studies found that in patients who had neuro-debilitating diseases like Parkinson's disease had a reduced P600 potential as a result of impaired basal ganglia.

Early researchers theorized that semantics and syntax involved in Wernicke's and Broca's area and thus they wanted to localize semantics and syntax processing onto these areas respectively.[9] Technologies such as electroencephalography (EEG), magnetoencephalography (MEG), and functional magnetic resonance imaging (fMRI) studies revealed that there were inconsistencies with this theory. Grodzinsky and Friederici (2006) examined previous research results that used MEG, fMRI, and EEG technologies and attempted to localize syntactic processing using previous results.

Grodinzsky and Friederici found that the frontal operculum, which is in the left inferior frontal gyrus and is adjacent to the inferior region of Broca’s area, was involved in understanding phrase structures. They also found that the anterior superior temporal gyrus was involved in processing structure violations, and it appeared that it was recruited in order to identify mismatching between the incoming sentence and the expected syntactic structure of the sentence. MEG studies revealed that these two structures were involved in local phase structure building but it appeared that the largest activation came from the left anterior super temporal gyrus, while a smaller activation was seen in the inferior frontal cortex.

Grodzinsky and Friederici saw that when sentences had complex syntax, there was an activation with Broca’s area (Brodmann’s area 44, 45) and they concluded that this activation was due to an increase on the working memory. This was confirmed through electrophysiological data. When integrating syntactic information, the left posterior superior temporal gyrus became active. Grodzinsky and Friederici theorized that this region was most likely used to support the integration of syntactic and lexical information between the left and right posterior superior temporal gyrus. This was supported through ERP measurements, with the discovery of a late centro-parietal positivity that occurred 600 ms (P600) after being presented with information.

The most important thing that Grodzinsky and Friederici concluded was that syntax processing actually occurred within the left interior frontal gyrus (IFG) and not Broca's area. Not only is the IFG responsible for syntax processing but other subdivisions are involved in different stages of syntax processing. See areas in syntax processing for further information.

Recent studies on aphasics have shown that while Broca’s area does seem to be associated with some aspects syntax processing, not all lesions are situated within Broca’s area.[10] Dronkers et al (1994) performed an experiment that examined morphosyntactic processing (i.e. structure of sentences) in people who had lesions, and in people who did not have lesions. They found that when patients had a low score on morphosyntactic tests, they typically have lesions on the left anterior temporal lobe (Brodmann’s area 22) rather than in Broca’s area. Also some patients who did have damage within Broca’s area had no sign of a severe syntactic deficits. It seems that Broca’s aphasics are capable of understanding the syntax for certain structures, until complex sentences are used that involve complicated word orders.

These results suggest that while Broca’s area is likely to be involved in syntax, it may only be restricted to complex sentence structures instead of various types of syntactic processing.[1] Other studies have shown that Broca’s aphasics may exhibit deficits in semantic processing as well, which is further suggestion that Broca’s area may not be solely involved in syntax processing. Different studies have utilized different neuroimaging techniques in order to compare and contrast different types of syntactic processes such as comparing complex sentences to simple, sentences to word lists, sentences with pseudowords compared to senseless sentences, and comparing sentences that have syntax violations to ones that do not. The following sections are based on a review paper by Kaan and Swaab.

When comparing sentences with complex syntax to simpler sentences, the assumption is that simpler sentences will not have as much syntactic operations as complex sentences, and thus will not activate areas within the brain as much as complex sentences do (Kaan and Swaab, 2002).[1] This assumption was tested by having subjects determine whether two sentences with different syntax had the same meaning, or if they had a separate meaning. It was found that there was enhanced activity in Broca’s area, more specifically with Brodmann’s area 44/45 in the left hemisphere. It was occasionally found that Brodmann’s area 6, 9, 21, 22, 23, 24,30, 31, 32, 39, and 47 would activate. Complex sentences consistently activated Brodmann’s area 44 and 45 but researchers were careful to ascertain that this did not mean that syntactic processing resided specifically within Broca’s area. This was because while these areas did activate while analyzing sentences with complex syntax there was another factor at play that may explain the activation of these regions: memory load. It appears that Broca’s area is activated when sentences contain ambigious words which supports the idea that Broca’s area is necessary for processing and memory for syntax.

Further evidence demonstrates the concept that Broca’s area is involved in processing load by the way of contrasting sentences to word lists.[1] After research compared complex sentences to simple sentences a problem arose: how do you tease out which activations occur specifically for syntax since both sentences use syntax? It was assumed that if researchers contrasted a sentence to a word list that they would be able to determine specific areas within the brain that were activated during syntax processing versus normal word activation. When comparing sentences that contained a syntactic structure to unrelated words in a list with no syntactic structure, it was found that Broca’s area was not significantly activated. The sentences used were not complex which is most likely why Broca’s area was not activated. Instead it was found that Brodmann’s area 38 would activate bilaterally, and it has been found that this area corresponds to the region responsible for patients who had morphosyntactic problems (see Dronkers et al, 1994). This implies that Broca’s area is involved in processing load rather than actual syntactic processing. These tests were not optimal for proving whether Broca's area was activated. This was because researchers were unclear that activation in Broca's area may be due to differences in semantic operations and syntactic structure between sentences and word lists (i.e. a sentence may have different semantic operations than word lists). Researchers reduced semantic processing through the use of a new test: pseudoword sentences vs senseless sentences.

In order to reduce semantic processing, pseudowords and senseless sentences were used. [1] Pseudoword sentences, also known as “Jabberwocky” words, are sentences that are grammatically correct but verbs, adjectives, and nouns are replaced by pseudowords. These pseudowords are phonologically and orthographically correct for the language being examined, but they lack meaning. Senseless sentences consisted of using existing words in grammatically correct sentences that made no sense. The concept of using Jabberwocky sentences and senseless sentences was that they may activate regions within the brain more than a normal sentence because they lacked semantic cues. The result of the experiment showed that Brodmann’s area 22, 38, 41, and 42 activated in Jabberwocky sentences.[11] It was found that a medial area of Broca’s area was activated through the use of Jabberwocky sentences compared to normal sentences, lists of pseudowords, and lists of real words. No frontal activation was found though so it was believed that the activation seen in Friederici et al (2000) study was perhaps due to task demands and not a result of syntactic processing.

The last process to determine whether or not syntactic processing was situated within Broca’s area involved studying sentences that contained syntactic violations to correct sentences (i.e. flowers can grew vs flowers can grow) or sentences that contained a different type of violation (i.e. flowers can run).[1] It was believed that when sentences violated syntax, areas involved in syntactic processing would become activated because the normal processing operations would become disrupted. It was found that sentences with syntactic violation did not typically activate Broca’s area but instead would activate superior frontal activity. Areas that were typically involved in semantic violations (Brodmann’s area 6, and 8) were activated but it was believed this may be because syntactic violations would have a consequence for the interpretation of sentences, thus activating regions involved in semantics. This was controlled for through the use of Jabberwocky sentences in other study, and it was found that in these cases Brodmann’s area 44 (right hemisphere) and 45 (left hemisphere) would activate. However it was found that Brodmann’s area 44 would activate for conditions that involved error detection which suggested that this region was not involved in just syntactic processing.

When it comes to syntax there are several different categories of syntax that have separate processing systems.[12] These different categories associated with syntactic processing are: word category information, gender information, and verb argument structure.

Studies involving ERP measurements have shown that word category information processing is processed before other syntactic information like gender information. Hahne and Friederici performed an experiment that involved participants listening to sentences that may be semantically incorrect, syntactically correct, syntactically incorrect, or both.[13] They found that when a word category violation has been made the brain processes this violation anteriorly in the left hemisphere around 150-200ms. However, when a gender information syntactic violation has been made, this is processed later (at 300-400 ms) within the same region.[14]

After ERP measurements demonstrated that different aspects of syntactic processing are processed separately, Heim et al attempted to localize these processing systems within the brain.

Heim, Optiz, and Friederici (2003) performed an experiment where they used fMRI scans to show that there was a difference in syntactic processing within Brodmann's area 44. (BA 44)[12] They designed an experiment in which subjects had to perform different decision tasks (i.e. determine whether the gender of a noun was masculine or neuter). The reaction time of the decision was measured via pressing a button. Since fMRI was being used, Heim et al were able to localize the different decision making tasks to distinct areas in the brain. They discovered that word category information and gender information were processed in separate regions of BA 44. Heim et al also discovered that different regions of Broca’s area were activated during specific syntactic tasks such as activation in BA 45 for gender information, and activation within BA 47 for word category information. Both of these regions are known through various research to be involved in semantic processing.

The IFG is a region of the brain which is found to be the most important aspect within a syntactic processing neural net.[15] The IFG is responsible for parsing. It has been postulated that when it comes to syntactic knowledge, the left anterior brain appears to be involved in this type of processing.[8] Friederici et al (2003) proposed that when it comes to syntactic processing there are two systems involved: an automatized initial process that is involved in the structure-building process, as well as a second system that kicks in later for a controlled process of syntactic repair and reanalysis.

Neuroimaging techniques have shown that Brodmann’s area (BA) 44 (which is the IFG) is responsible for syntactic processing.[12] Friederici, Mayer, and von Cramon (2000) performed an experiment using event-related fMRI to study processing of single words.[16] The purpose of using single words (that varied in their syntactic and semantic status) was to determine which tasks activated what areas. They discovered that syntax processing within BA 44 can be broken down into other components. They found that the inferior portion of BA 44 is actually responsible for processing local structure building and word category information while the mid portion of BA 44 is involved in syntactic memory.

Other areas involved in syntax processing are the cingulate gyrus, the left superior frontal gyrus, the left caudate nucleus, the middle and superior temporal lobes, the anterior temporal lobe, the posterior temporal area, and the right hemisphere of the brain.[12][1] White matter pathways that seem to be involved are the dorsal pathways: the arcuate fasciculus and the dorsal pathway that runs from the superior longitudinal fasciculus to the posterior temporal lobe. [17] When it comes to human language, it seems as the though the latter white matter pathway is particularly important.

One model proposed by Kaan and Swaab (2002) is that different regions within the brain are linked together to form a network for syntactic processing.[1] These regions are the middle and superior temporal lobes which may be responsible for activating semantic, phonological, and syntactic information as well as involved in lexical processing. The anterior temporal lobe may be responsible for encoding information to be used at a later point, or in combining the activated information from other regions within the brain. Broca’s area is likely to be responsible for storing large amounts of information that is non-integrated when the sentences become more complex, a syntactic working memory. Broca’s area may also activate the posterior and middle temporal areas to feed back lexical information, or may be involved in recruiting the visual working memory as a way to store information. The right hemisphere of the brain is also activated during syntactic processing in order to process tone, decipher ambiguity, syntactic violations, and discourse processing.

Friederici in 2009 proposed a new model that involved the language areas of the brain (The superior temporal gyrus (STG), middle temporal gyrus (MTG), the IFG). [17] Her model incorporated both gray and white matter as a way to explain language processing. Friederici included white matter in her model because the areas between the brain need a way to communicate between one another, and white matter fiber bundles do this through connections to adjacent and distant brain regions. Friederici believes that the arcuate fasciculus is the white matter pathway between the IFG and the STG, but research remains inconclusive. Through fMRI and other imaging studies, other white matter pathways have been identified such as a dorsal pathway that runs from BA 44 via the superior longitudinal fasciculus to the posterior temporal lobe. This pathway also contains connections to BA 40, the lateral middle temporal gyrus and superior temporal gyrus, as well as other ventral routes that connect to Broca’s area. Two ventral pathways also exist: the extreme capsule and unicinate fasciculus which connect to the anterior superior temporal gyrus. While her model focuses on how language was processed rather than just syntax, it provides valuable information that was previously unknown. The model does mention that when children are younger they should have problems with processing complex syntactic sentences. This is because the dorsal pathway that connects language areas is not yet fully myelinized within children. Evidence (behavioural and functional imaging) does support that the dorsal pathway for children is not fully myelinized and that this weak myelination may be responsible for deficits in syntactic processing.

Syntax processing arose out of Broca and Wernicke's discoveries which were the first stepping stones of real science that began to localize syntax within the brain. As technologies evolved and grew, the search for syntax began to as well. ERP's led researchers to the conclusion that syntax was a separate feature of language processing, and that it was different from semantic processing. The results of ERPs led scientists to other technologies like MEG and fMRI scans to try and pinpoint where in the brain syntax processing occurred. Scientists made the surprising study that what was classically called the seat of syntax (Broca's area) was actually not involved in syntax processing but instead was used for memory load when sentences became complex. As researchers designed and created new experiments, they began to tease the different faculties of syntactic processing apart from one another and realized that syntax processing was a complicated procedure that involved many different processing systems such as structure building, gender information, etc. All of these different processing systems localize to different regions with the brain, some of which are subdivided into further categories (i.e. the IFG) that are involved in syntax processing.

I hope that after reading this chapter you now have some idea as to what syntax is, how it works, and where in the brain syntax functions. Syntax seems like a simple process but when you delve into the nitty gritty details about where syntax actually functions in the brain and how it communicates to other aspects of language processing, you begin to realize the huge amount of power that syntax processing harnesses.

Compare and contrast Kaan and Swaab's model of syntactic processing with Friederici's model of syntactic processing.

What evidence originally suggested that syntax processing was not localized in Broca's area?

What evidence suggested that syntactic processing and semantic processing were separate from one another?

How did Broca's research coincide with the basic concepts of phrenology? How do recent findings fit with the concept of phrenology?

How did people think language processing occurred in the 19th century? How did Broca and Wernicke's discoveries contribute to our idea of language processing? How do these old findings fit in with more modern science?

What are the pros and cons of Kaan and Swaab's model of syntactic processing, as well as Friederici's model of syntactic processing?

If you wanted to design an experiment to test for syntactic processing, how would you model this experiment? What results would you expect to find?

If you suffered from a lesion that affected the area in your brain responsible for word memory load, what area(s) would be affected and what kind of deficits would you have?

If you were to build a "house" out of syntactic processes, what "pieces" would you require and what order would you "lay them down" to complete your house?

If you designed an experiment to test syntactic processing and you presented sentences in a subject-verb-object order what would you expect to happen? What do you think would happen if you switched the order?

Construct a flow chart of syntax. For example, what information would be processed first and in what area of the brain? Continue until you've laid down the foundation of syntax processing in the brain.

If you had to contribute to an artificial intelligence computer which was meant to store language if something happened to the human race, what aspects of language would you determine were important for syntax processing? How would you organize the different syntactic processes so that they could function approximately like they would within a human being? (Hint: Think about what areas you would need, and what processes need to build on one another.)

What areas in the brain do you think are important for syntactic processing?

It was believed by phrenologists that language occurred underneath the eye. According to phrenologists, what job would be best suited for a person with a protrusion beneath the eye?

Based on what you have learnt from this chapter construct your own syntactic processing model.